1. Field of the Invention
The present invention relates to a method of correcting chromatic aberrations in a charged-particle beam and to a charged-particle beam system.
2. Description of Related Art
A scanning electron microscope is described as an example of specimen surface imaging instrument using charged particles by referring to FIG. 5, which depicts an example of configuration of the prior art instrument. This prior art instrument is equipped with an aberration corrector. An emitter 1 emits an electron beam 2 toward a chromatic aberration corrector 4. The beam 2 hitting the corrector 4 is controlled by a condenser lens 3 acting on the beam 2. The beam 2 is focused onto a surface of a specimen 6 by an objective lens 5 from the chromatic aberration corrector 4.
The electron beam 2 is scanned over the surface of the specimen 6. Secondary electrons 7 ejected from the surface of the specimen 6 are detected by a secondary electron detector 8 in synchronism with the scanning. Thus, the secondary electrons are displayed as an image on a display device 10 in synchronism with the scan signal. Since the detection efficiency for secondary electrons is normally low, the output signal from the detector is accumulated by an image accumulator 9. This enhances the ratio of the intensity of the signal component to noise intensity. Known examples of the aberration corrector are described, for example, in the following references: J. Zach, International Patent Application Number WO 01/56057 A1 (2000); V. H. Rose, Optik 33, Heft 1, 1(1971); J. Zach, Optik 83, No. 1, 30 (1989); and J. Zach and M. Haider, Nucl. Instr. and Meth. in Phys. Res. A 363, 316 (1995)
A chromatic aberration corrector using multipole elements described in the J. Zach, V. H. Rose, and J Zach et al. references which constitute well-known techniques of this kind is now described as an example. A chromatic aberration corrector corrects chromatic aberrations in the whole system. However, it is difficult to judge whether the aberrations have been actually corrected from the SEM image (scanned image), because the electron beam 2 is focused to some extent on the specimen surface even if aberrations are contained. In the above-cited well-known techniques, the operator judges the sizes of chromatic aberrations from the SEM image displayed on the display device 10 and manually manipulates an aberration corrector controller 12, thus correcting the chromatic aberrations. The prior art method of manually correcting chromatic aberrations is described below.
Electrons are ejected from the emitter 1 at various accelerations. The energies are distributed from about 0.2 to 0.9 eV at a field emission emitter or thermal field emitter. When an electron having an average energy is focused onto the specimen surface, an electron having an energy deviating by ΔE is focused to a position slightly deviating from the specimen surface in the direction of travel of the electron beam by an amount Δf. If chromatic aberrations are taken into account up to the second order, the amount of deviation Δf is given byΔf=Cc ΔE+Kc ΔE2  (1)where Cc and Kc indicate the chromatic aberration coefficients of the first and second orders, respectively, of the whole system for energy dispersion. Defocus given by Eq. (1) and due to each ΔE is combined with the distribution of ΔE. Because of the resulting effect, the SEM image is observed as a blurred image. Since the amount of the blur is quite small, it is difficult to judge whether the chromatic aberrations have been corrected, by observing the SEM image.
Furthermore, the sign of Cc cannot be known only from the image blur. Accordingly, the potential at the emitter 1 is shifted by several volts by an energy shift controller 11 to shift the whole distribution of ΔE. As a result, the whole system is underfocused or overfocused. Hence, it is possible to judge the sign of Cc.
When the emitter potential is varied by +ΔE by the energy shift controller 11, the image is blurred. It is now assumed that the strength of the objective lens 5 is varied by the objective lens controller 13 and that the image can be brought to a focus. Defocus Δf1 can be known from the variation in the strength of the objective lens 5 occurring at this time. Similarly, defocus Δf2 occurring when the emitter potential is varied by −ΔE can be known. Accordingly, from Eq. (1), Cc produced at this time is given byCc=(Δf1−Δf2)/2ΔE  (2)
The aberration corrector controller 12 is so manipulated that the value of Eq. (2) becomes equal to 0. In this way, the amount of correction to the chromatic aberration is adjusted. The chromatic aberration corrector 4 is made up of multipole elements. As described in the above-cited references, corrections can be made in the x- and y-directions independently. Accordingly, it is necessary to carry out the above-described procedure in the x- and y-directions.
The procedure for aberration correction is complex as mentioned above. There is the problem that considerably long time is required for an ordinary operator to master the technique for obtaining high-resolution images.